Optical printing devices for printing on blanks which are intended for swaging

ABSTRACT

PCT No. PCT/FR83/00221 Sec. 371 Date Jun. 13, 1984 Sec. 102(e) Date Jun. 13, 1984 PCT Filed Nov. 8, 1983 PCT Pub. No. WO84/02008 PCT Pub. Date May 24, 1984.A device is disclosed for printing on blanks which are intended for swaging, to obtain a predeformed image on the blanks which, following swaging, will reconstitute the original image. The device comprises a light source (2), a toric prism (4) and a camera (8) having a common optical axis (12). The angle of the toric prism is smaller than 42.05 DEG , and the surface (11) of the prism facing the camera (8) forms a linearity correction lens.

The invention concerns an improvement to be applied to optical printingdevices for printing on blanks which are intended for the manufacture ofcans by swaging.

Cans are decorated either by means of a strip of paper which ispreprinted and wound around and glued onto the can, or by directprinting on the can, or else by printing of the blank which is then toproduce the can by swaging.

In this case, the blank generally must be printed with a predeformedimage in such a manner that the original image will be reconstitutedfollowing the swaging.

Optical apparatuses to obtain a predeformed image have been described inprior documents, particularly in U.S. Pat. Nos. 3,238,909, 3,314,329,3,964,910, 4,119,484 and in FR Pat. No. A-1 590 126 (corresponding toU.S. Pat. No. 3 627 412) in the name of the applicant, as well as in FRPat. No. A-2 453 432, in the name of FEREMBAL, which uses a flexiblereflecting strip as optical element, turning on the axis of the opticalsystem around the can which is being formed.

Presently, the requirements of users regarding the quality of printing(accuracy, contrast, restoration of colors, etc.) are no longer entirelysatisfied by known optical devices; the legibility of texts in smallcharacters, for example, is not always assured. In four-color printing,a lack of clarity has been noted, which requires stopping down ofdiaphragm aperature so as to increase the depth of the field, thusincreasing the exposure time; it was then noted that the diffused lightattenuates the shadows in the vicinity of the light zones, which gives a"milky" appearance to the print.

The object of the present invention is an improvement of the anamorphicoptical system, which is the object of our French Pat. No. FR-A-1 590126. In this patent, a printing apparatus is described for printing apredeformed image on swaging blanks, characterized in that it comprisesessentially: a toric prism and an aspherical and revolving lens,associated with an achromatic photographic objective of very great focallength.

The toric prism is realized with a very transparent material such asmethyl polymethacrylate.

The improvement consists of associating the toric prism and thecorrection lens in one monolithic element, the prism solely assuring thefunction of reflection of the light rays, and the lens effecting theoptical corrections assuring the linearity of the swaged image. Also,the angle of the prism has been determined in such a manner as to obtaintotal reflection of the light rays emitted by the source.

FIGS. 1 to 8 illustrate the application of the invention, mainlyillustrating the general principle of the invention.

When a can is swaged from a flat, circular blank, it is noted that eachpoint marked on the flat surface of the blank then corresponds to onepoint on the surface of the swaged can. A relation thus exists betweenthe prior position, on the blank, of any one point of the surface of themetal, and the final position on the swaged can. Then by knowing thisrelation, it is possible to print the blank before deformation in such amanner that during the swaging the drawing is subject to a deformationsuch that the desired picture is obtained. In a practical manner, alabel which can be wound around the can is generally used; the blankshould then be printed in such a manner that, following swaging, a canis decorated with the same picture as if the label had been wound aroundit.

If the conversion which makes the points of the blank correspond tothose of the can is T, then:

blank→(T)→can

and the process consists of optically making the inverse conversion T⁻¹so as to obtain a deformed image from a label, to be used for theprinting of the blanks:

label→(T⁻¹)→blank.

By starting with a lable, and proceeding with these conversions, a canis obtained which is truly of the same appearance as the label:

label→(T⁻¹, optic)→blank→(T, swaging)→can.

To determine the function I, a printed blank is swaged which has a gridof lines forming a system of coordinates to mark the position of a pointboth before and after swaging.

In the case of round cans, a system of polar coordinates (r, θ) is usedfor the blank, and the position of the points on the can after theswaging is then marked by a system (p, θ) wherein p is the position of apoint measured on a generatrice of the can from the bottom. It is thenpossible to consider that the conversion T is summarized in a relationbetween r and p. In fact, it is preferable to consider the valuet=r-r_(o), wherein r_(o) is the radius of the can, and one then has arelation between the number of lines (in mm) and their position on theoutside of the can.

It must be noted that the conversion T which is thus obtainedexperimentally by plotting depends upon the conditions of the experiment(quality of the metal, the grid lines, and condition of the tools,etc.). Thus, to subsequently obtain good results, one must repeat thisexperiment in conditions nearest those which will be used in industrialpractice. One must therefore swage a sufficient number of cans with thedefinitive metal to reproduce the conditions of an assembly lineproduction, and one must also pay attention to the centering of theblank which carries the system of coordinates r, θ (r varying from mm tomm).

The final result is then summarized in a table giving the values of pcorresponding to each of the values of t. This table, completed by adimensional summary survey of the can, allows realization of the opticalsystem effecting the conversion T⁻¹.

FIG. 1 shows a longitudinal cross section of the optical devicedescribed in FR Pat. No. A-1 590 126.

FIG. 2 is a diagram of the conversion by swaging of a point t of theblank into a point p of the can.

FIGS. 3 to 7 show the passage of the light rays in the optical device,object of the invention.

FIG. 8 shows the cross section of a prism realized according to theinvention.

The light rays (1) emitted from the source (2) pass through thetransparent plate (3) which carries the image to be reproduced, thenpasses through the toric prism (4) of which the outside is silver- oraluminum-coated. The rays then pass through the lens (6) and thediaphragm (7) of the camera (8) and on the sensitive flat lens surface(9) come to form the anamorphic of the image of plate (3).

In FR Pat. No. A-1 590 126, the optical corrections were provided by anadditional lens (10) shown in broken line, and placed at a certaindistance in front of prism (4).

According to the present invention, this additional lens (10) isdeleted, and the shape of the surface (11) of toric prism (4) facing thecamera is modified to assure the optical corrections. The common opticalaxis of the system is shown by line (12). Besides, prism (4) ispreferably of methyl methacrylate, and silver- or aluminum-coating ofits outside (5) is difficult to realize and remains very fragile. Toovercome this drawback, the angle of the prism, which was originally45°, is modified in such a manner that the light rays are subject tototal reflection. As a modification of the invention, it is alsopossible to first realize the prism-lens correction assembly in twopieces and then to combine them in one monolithic block, preferablywithout using any connector such as glue, by very finely polishing thetwo contact surfaces (front surface of the prism, rear surface of thelens), so that there is perfect contact between the two surfaces withoutinterposition of a layer of air.

This variation allows for an assembly of interchangeable lenses cutaccording to the different profiles to take into account the particularfeatures of each type of can to be printed; it also allows easiercutting of the lens, because the toric prism is not easily fixed on thetooling machine.

The angle of the prism is determined in the following manner:

H the height of the prism,

h the height of the can (i.e. of plate 3),

Φ_(o) the inside diameter of toric prism (4),

F the focal length of the lens (6),

A the staggering of the base of plate (3), representing the drawing tobe reproduced, in relation to the edge (13) of the prism, because it isnot possible to have h=H, as the last 3 or 4 millimeters of the end andtop parts of the prism cannot be used, in practice.

The critical angle of the methyl polymethacrylate (of which therefraction index is n=1493) is: ##EQU1##

Thus it is necessary to choose an angle smaller than 42.05° to assuretotal reflection. The best results are obtained with α<35°, and,preferably between 35° and 32°.

Let α be the angular value of which the angle is decreased from 45° fortotal reflection. The end reflection point is that which corresponds tothe top of the can (crimping area).

Let h_(o) be this height: ##EQU2##

(N.B.: The I/O optical enlargement is only slightly different from 1,from whence the division of 2F).

α can be determined very approximately by allowing that tg (45-α) #1,and thus alpha #0, wherein: ##EQU3##

NUMERIC APPLICATION AND COMPUTATION OF THE VALUE OF A

One takes Φ_(o) =68 mm, h_(o) =28 mm. H=60 mm is set to be sure that allof the rays are reflected. The focal length of the objective is F=480mm.

(N.B.: the value of F=480 mm is given as a nonlimiting example).

One has: ##EQU4##

which can be rounded to 3 mm, according to the results of experiments.##EQU5##

The angle of the prism thus must be less than: 45°-3.87°=41.17°.

For safety, a prism of 35° angle will be adopted. It is now possible,from this data, to calculate the precise value of A.

One has: ##EQU6##

According to FIG. 4, one has:

    tg α=x/H                                             (g)

Because of the reduction of the angle of the prism from 45° to 35° thevirtual image of the plate, which was formed on the plane P, is nowformed on a cone C which forms an angle of 2=20° (FIG. 4) with the planeP. ##EQU7## and consequently ##EQU8##

For d=1 mm, one has: ##EQU9## with F=480 mm

Φ_(o) =68 mm

h_(o) =28 mm

H=60 mm

is found:

A=3,325 mm

A=5 mm will be adopted in practice.

COMPUTATION OF THE CORRECTION TO BE APPLIED TO SURFACE (11)

It now has to do with the calculation of the shape of surface (11) ofthe prism to assure the optical corrections. The plane of virtual imageP has become a cone C of virtual image, and the projection of this coneon the vertical plane (FIGS. 5 and 6) is going to be considered withreference to a similar case.

The degree of enlargement γ, which, in the case of a flat surface (11)and an α angle=45°, for a given point, would be: ##EQU10## as a resultof the projection becomes: ##EQU11##

In accordance with the formula of the centered systems ##EQU12## whereinr is the staggering of the plane of the virtual image given by theprism+surface playing the role of lens system, and consequently:##EQU13## wherein l is the distance between the edge of the prism andthe objective (FIG. 5). To determine r, one must know the optical pathpassed through by the ray in the methyl polymethacrylate (FIG. 6).

When the advance of different rays in the prism is represented, it isclear that r varies as a function of p (defined above, as the positionof a point measured on a generatrice of the can from the bottom). Butthis value of r depends upon the incidence of the ray on the surface,this being known only when its coordinates x and y are calculated. Thatis why r is determined approximately in the following manner: ##EQU14##Therefore, one has (FIG. 7): ##EQU15##

The approximation of the surface for the prisms with small angles givesthe following: ##EQU16## with δ=displacement in the vertical projectionof the virtual cone. ##EQU17##

Also, if the angle of the tangent to the surface of the lens is calledβ: ##EQU18## then, consequently: ##EQU19## knowing that l'=l-H ##EQU20##

x_(t) and y_(t) are therefore parametric coordinates which define theform of the surface (11) of the prism, cut as a corrective lens, and theaxis y is the optical axis (12) of the system. The reference plane (x,y) is selected arbitrarily. This can be the surface (11) of the prismbefore cutting, or any other plane, parallel to this surface, forexample defined by the value H.

The use of the coordinates x_(t), y_(t) to define the front surface ofthe prism will easily lend itself to working by digital control.

NUMERIC APPLICATION

Starting from φ_(o) =68 mm; A=5 mm; F=480 mm; n=1493 mm; H=60 mm and2α=20°, a certain number of parameters C₁ to C₄ are first determined.

Number of C₁ to C₄ parameters: ##EQU21##

y_(o) =0 (which corresponds to the reference surface of the prism)

x_(o) =C₁. C₃ (=35.23),

and then for each pair (p, t) is calculated: ##EQU22## Recalling that thas been defined above, in relation to the polar coordinates (r, θ), ast=r-r_(o), r_(o), being the radius of the can, (see FIG. 2), one has:##EQU23##

PRACTICAL EXAMPLE OF COMPUTATION OF THE FRONT SURFACE OF THE LENS

A can carrying a system of polar coordinates and obtained by swaging isconsidered.

The values of p correspond to the values of t on the blank (FIG. 2). Thevalue t=0 is given in the first visible circle on the bottom of the canplaced on a face plate. p=0 is associated with t=0, from which originatethe measures. For a can of H=28 mm, it is possible to set 20 values fort/p pairs.

On a can of 68 mm diameter and 28 mm height, with an objective of 480 mmfocal length, the parameters x_(t) and y_(t) are obtained for each t/ppair.

                  TABLE I                                                         ______________________________________                                        t      p              x.sub.t y.sub.t                                         ______________________________________                                         0     0              36,2441  0,0000                                          1      1,02          37,3057 -0,0080                                          2      2,14          38,3665 -0,0191                                          3     3,1            39,4279 -0,0411                                          4      4,36          40,4876 -0,0591                                          5      5,58          41,5471 -0,0752                                          6      6,86          42,6061 -0,0865                                          7      8,04          43,6653 -0.0978                                          8      9,26          44,7239 -0,1072                                          9     10,4           45,7827 -0,1186                                         10     11,76          46,8400 -0.1214                                         11     13,08          47,8972 -0,1176                                         12     14,42          48,9540 -0,1063                                         13     15,76          50,0104 -0,0874                                         14     17,16          51,0661 -0,0584                                         15     18,56          52,1215 -0,0192                                         16     19,94          53,1767 -0,0291                                         17     21.26          54,2319 -0,0838                                         18     22,52          55,2871 -0,1418                                         19     23,94          56,3410 -0,2107                                         20     24,8           57,3983 -0,2647                                         ______________________________________                                    

Starting from parameters x_(t) and y_(t), it is now possible todetermine the coordinates X and Y defining the shape of the lens, fromthe formulas I and II.

The values, calculated on computer, are indicated in Table II. The frontsurface of the lens has a 140 mm diameter, but the shape applies only toa diameter of 120 mm. The profile has been determined from 25 points forwhich the values of X are given along the radius of the lens and Y (sideof the shape).

                  TABLE II                                                        ______________________________________                                                    X     Y                                                           ______________________________________                                         1             0,0000  0,0000                                                  2            10,0000  0,0000                                                  3            20,0000  0,0000                                                  4            34,0000  0,0000                                                  5            36,2441  0,0000                                                  6            37,3057 -0,0080                                                  7            38,3665 -0,0191                                                  8            39,4279 -0,0411                                                  9            40,4876 -0,0591                                                 10            41,5471 -0,0752                                                 11            42,6061 -0,0865                                                 12            43,6653 -0,0978                                                 13            44,7239 -0,1072                                                 14            45,7827 -0,1186                                                 15            45,8400 -0,1214                                                 16            47,8972 -0,1176                                                 17            48,9540 -0,1063                                                 18            50,0104 -0,0874                                                 19            51,0661 -0,0584                                                 20            52,1215 -0,0192                                                 21            53,1767  0,0291                                                 22            54,2319  0,0838                                                 23            55,2871  0,1418                                                 24            56,3410  0,2107                                                 25            57,3983  0,2547                                                 ______________________________________                                    

The use of a prism, of which surface (11) has been shaped as indicatedabove, has furnished clear prints, contrasted and without geometricerrors. In particular, the tests carried out with exposures on Kodak"Ektachrome" and screens of 133 lines per inch have shown an excellentcolor restitution, absence of any diffusion and parasitic lightreflections, and excellent contrast of the image and an absence ofdeformation of the screen.

I claim:
 1. Device for the printing of blanks which are intended forswaging, to obtain a preformed image on the blanks which, followingswaging, will reconstitute the original image, comprising a lightsource, a toric prism, and a camera having a common optical axis,theangle of said toric prism being less than 42.05°, and said toric prismhaving a surface facing said camera forming a linearity correction lens,the cutting of said lens-forming surface being defined, in a system ofparametric coordinates x_(t) and y_(t) centered on the intersectionpoint of the optical axis, and by a reference plane parallel to saidsurface of the prism, defined by the equations: ##EQU24## the C₁, C₂,C₃, and C₄ parameters being defined from the inside diameter Φ_(o) andthe height H of toric prism, from the offset A of the plate to bereproduced in relation to the edge of the prism, from the focal length Fof the lens of camera and from the refraction index n of the materialforming the prism and from the top angle (45 -α) of said prism. 2.Printing device as in claim 1, wherein the angle of said toric prism isbetween 35° and 32°.
 3. Printing device as in claim 1 or 2, wherein thelens and the prism are obtained by simultaneous cutting in one singlesystem.
 4. Printing device as in claim 1 or 2, wherein the lens and theprism are obtained by separate cutting and very fine polishing of thefacing surfaces, which are then placed in contact so as to form amonolithic block.